Catch a Comet

The night sky can be full of wonderful surprises. Just ask Alan Hale, who works as an astronomer in New Mexico. Or Tom Bopp, who lives near Phoenix and works for a company that sells construction materials. One July night in 1995, they both decided to look through their telescopes.

While looking at a cluster of stars, they noticed a tiny fuzzy blob that wasn't there the last time they were stargazing. Mr. Bopp and some friends watched the object for an hour, while Dr. Hale tracked it for three hours. Both men noticed that the fuzzy patch of light seemed to move across the sky faster than the stars did. They looked at special astronomy books to see if the object had been listed before. It hadn't. They quickly reported their discovery - a new comet. It now carries their names: Comet Hale-Bopp.

In a few days, Hale-Bopp will become visible in the morning sky, about an hour before dawn. Astronomers say the best viewing may come in late March about an hour after sunset. Either way, you don't have to be an astronomer with a fancy telescope to view this astronomical treat. With a pair of binoculars, or even just a sharp pair of eyes, you'll be able to follow this visitor from the far reaches of our solar system as it swings past the sun and heads back into deep space.

At first glance, you may see a fuzzy patch of light (the comet's "atmosphere" or coma) with a bright center and a tail. What you're really seeing is an orbiting fossil from the earliest days of our solar system. Scientists say comets are leftovers from the vast cloud of dust and gas that gave rise to the sun and planets.

Scientists have learned a lot about comets just by studying them with telescopes. But their best view came in 1986. A spacecraft named Giotto passed within 370 miles of Halley's comet, which orbits the sun every 86 years. When scientists saw the data Giotto sent them, they were thrilled. The up-close-and-personal look confirmed their notion that comets are "dirty snowballs" made of ice, dust, and rock.

In 1999, scientists hope to send a spacecraft to a comet and have it return with samples of the dust. And in 2003, they hope to study another comet with a craft that would orbit the nucleus and send probes to land on it.

Hale-Bopp's nucleus is thought to be from six to 25 miles across. As the comet nears the sun, its nucleus heats up. The ice turns directly into a gas without becoming a liquid first. And the comet develops a tail - a thin trail of gas that gives off a blue glow. The glow comes from the sun's radiation interacting with the gas. When the

comet gets really close to the sun, the tail may brighten as sunlight reflects off the dust the comet leaves behind. When comets cross Earth's orbit, debris from the comet can become spectacular meteor showers as tiny bits of rock or dust plunge into our atmosphere.

Hale-Bopp last came through our neighborhood 4,200 years ago. Astronomers don't expect it to return for another 2,400 years. So now's the time to see it.

Create your own comet journal

In fact, you could do more than just watch it! Try making a comet journal. Note the date and time you first observe the comet. Try to view the comet at the same time before sunrise or after sunset each day the weather permits. You can usually find the times for sunrise and sunset in your local newspaper. The best viewing is an hour before dawn and an hour after sunset.

Here's what you could do:

* Measure the comet's position as often as you can. Make one observation each day, or, if you can, take several for as long as the comet is visible.

Use a well-marked compass to find the direction you must face to see Hale-Bopp (its "azimuth"). And you could also make a simple astrolabe (see "Kidspace," Sept. 17, 1996 Monitor, Page 16) to measure the comet's position above the horizon (its "elevation"). Draw a chart showing how these two things change with time and compare them with the predictions in sky charts from magazines like Sky and Telescope or Astronomy.

* Estimate its brightness. Use a star finder to help you compare the comet's brightness with nearby stars. How does the comet's brightness change over time?

* Take photos, using fast (400 ASA or higher) film and a tripod. Experiment with different exposure times, say, from a few seconds to a couple of minutes.

This information not only can serve as a diary of a comet to look back on years from now; you could also use it as a project for your next school science fair!

The World Wide Web has some great Hale-Bopp sites. For starters, try these:

http://encke.jpl.nasa.gov/hale_bopp_info.html

http://www.halebopp.com/z01mag.html

Where's Hale-Bopp?

If Hale-Bopp is anything like last year's comet, Hyakutake, it will look like a light fuzzy patch with a bright center and a fainter tail. (Many astronomers think it will be brighter than Hyakutake was.) Finding a comet in the night sky can be a bit like scanning a page in a "Where's Waldo?" book. It gets easier with practice and by following these viewing tips:

1. Dress warmly; wear several layers of clothing. Late winter and early spring evenings and early mornings can be chilly.

2. Use a flashlight covered with red cellophane to help you find your way or to allow you to read a star chart or make drawings of what you see. Red light is easier on your night vision.

3. If you can, view from a flat open area or from a hilltop away from city lights. The sky will look darker, allowing the comet to stand out better. And you'll have a better view when the comet is low on the horizon.

4. Give your eyes about 20 minutes to adjust to the dark. Protect your night vision by not looking at any bright lights, even distant ones.

5. If you're in a field that's private property, get permission to use it ahead of time. If it's public property, like a park with baseball diamonds or soccer fields, take along this article or something else so that if police stop by, you can show them what you're doing. (Invite them to take a look!)

6. Don't forget a pair of binoculars. Although you can see it now, Hale-Bopp should be at its brightest from late March to early April. Binoculars should give you great views, and you'll be able to see more of the comet at once than you would if you use a telescope.

7. Take some munchies. (This needs a reason?)

Make a Comet For the Class

SCIENTISTS think that comets contain some of the original material from which the solar system formed. The recipe below contains actual components (or handy stand-ins) of comet material, based on ground observations, spacecraft fly-bys, and the collection of tiny ice particles by very high-flying research aircraft.

Note: Dry ice is minus 110 degrees F. Handle it carefully, and only with waterproof, insulated gloves. It's available from ice companies and ice-cream parlors. Long sleeves and goggles are good ideas, too.

You will need:

5 lbs. dry ice (to provide carbon dioxide, or CO2)

1/2 gal. of water in a pitcher

A few drops or sprays of window cleaner (for ammonia)

A couple pinches of cornstarch (carbon-based molecules)

A handful of dirt, fine-grained (silicates)

2 trash bags

A large bowl or pot

Waterproof, insulated gloves

Cloth towel

Hammer

Mixing spoon or stick

Line the bowl with a trash bag. Put second trash bag on floor. Pour a pint of water into bowl; add cornstarch, ammonia, and some dirt. Stir a bit.

Put on gloves. Wrap dry ice in cloth towel; place atop trash bag on floor. Hammer dry ice into powder. Gradually pour dry-ice powder into the water, mixing as you pour. Lots of vapor will form. Ingredients should form thickening slush. Stir for a few seconds.

Now use the trash bag to lift slush away from sides of bowl; use gloved hands on outside of trash bag to pack slush into a ball. Keep packing and forming until the ball solidifies.

Peel back trash bag. Scatter more dirt over the lump. Pour more water over lump, turning it so the water forms a layer of ice over the entire lump.

Watch how this miniature comet nucleus behaves. (If water-ice layer is intact, lump can be handled without gloves. If a spot feels sticky, pour water on it.) The nucleus hisses and pops as CO2 shifts directly from solid to gas and breaks through the water-ice crust. On real comet nuclei, this results in jetting forces that can cause the nucleus to spin, slightly altering its orbit, or to split apart. These escaping gases also mix with dust to form a comet's tail.

Note: The originators of this comet 'recipe' say that it's fun, but messy. If you plan to do this, practice first so you'll know about how much water to use. Dry ice can be purchased the afternoon before and kept in the freezer or an ice chest. Put an inch or so of newspaper under the dry ice to keep it from cracking the surface it rests upon.